On 12/02/2015 03:02 PM, [email protected] wrote:
Waiman Long<[email protected]> writes:If a system with large number of sockets was driven to full utilization, it was found that the clock tick handling occupied a rather significant proportion of CPU time when fair group scheduling and autogroup were enabled. Running a java benchmark on a 16-socket IvyBridge-EX system, the perf profile looked like: 10.52% 0.00% java [kernel.vmlinux] [k] smp_apic_timer_interrupt 9.66% 0.05% java [kernel.vmlinux] [k] hrtimer_interrupt 8.65% 0.03% java [kernel.vmlinux] [k] tick_sched_timer 8.56% 0.00% java [kernel.vmlinux] [k] update_process_times 8.07% 0.03% java [kernel.vmlinux] [k] scheduler_tick 6.91% 1.78% java [kernel.vmlinux] [k] task_tick_fair 5.24% 5.04% java [kernel.vmlinux] [k] update_cfs_shares In particular, the high CPU time consumed by update_cfs_shares() was mostly due to contention on the cacheline that contained the task_group's load_avg statistical counter. This cacheline may also contains variables like shares, cfs_rq& se which are accessed rather frequently during clock tick processing. This patch moves the load_avg variable into another cacheline separated from the other frequently accessed variables. It also creates a cacheline aligned kmemcache for task_group to make sure that all the allocated task_group's are cacheline aligned. By doing so, the perf profile became: 9.44% 0.00% java [kernel.vmlinux] [k] smp_apic_timer_interrupt 8.74% 0.01% java [kernel.vmlinux] [k] hrtimer_interrupt 7.83% 0.03% java [kernel.vmlinux] [k] tick_sched_timer 7.74% 0.00% java [kernel.vmlinux] [k] update_process_times 7.27% 0.03% java [kernel.vmlinux] [k] scheduler_tick 5.94% 1.74% java [kernel.vmlinux] [k] task_tick_fair 4.15% 3.92% java [kernel.vmlinux] [k] update_cfs_shares The %cpu time is still pretty high, but it is better than before. The benchmark results before and after the patch was as follows: Before patch - Max-jOPs: 907533 Critical-jOps: 134877 After patch - Max-jOPs: 916011 Critical-jOps: 142366 Signed-off-by: Waiman Long<[email protected]> --- kernel/sched/core.c | 36 ++++++++++++++++++++++++++++++++++-- kernel/sched/sched.h | 7 ++++++- 2 files changed, 40 insertions(+), 3 deletions(-) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 4d568ac..e39204f 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -7331,6 +7331,11 @@ int in_sched_functions(unsigned long addr) */ struct task_group root_task_group; LIST_HEAD(task_groups); + +#ifdef CONFIG_FAIR_GROUP_SCHED +/* Cacheline aligned slab cache for task_group */ +static struct kmem_cache *task_group_cache __read_mostly; +#endif #endif DECLARE_PER_CPU(cpumask_var_t, load_balance_mask); @@ -7356,6 +7361,7 @@ void __init sched_init(void) root_task_group.cfs_rq = (struct cfs_rq **)ptr; ptr += nr_cpu_ids * sizeof(void **); + task_group_cache = KMEM_CACHE(task_group, SLAB_HWCACHE_ALIGN);The KMEM_CACHE macro suggests instead adding ____cacheline_aligned_in_smp to the struct definition instead.
The main goal is to have the load_avg placed in a new cacheline separated from the read-only fields above. That is why I placed ____cacheline_aligned after load_avg. I omitted the in_smp part because it is in the SMP block already. Putting ____cacheline_aligned_in_smp won't guarantee alignment of any field within the structure.
I have done some test and having ____cacheline_aligned inside the structure has the same effect of forcing the whole structure in the cacheline aligned boundary.
#endif /* CONFIG_FAIR_GROUP_SCHED */ #ifdef CONFIG_RT_GROUP_SCHED root_task_group.rt_se = (struct sched_rt_entity **)ptr; @@ -7668,12 +7674,38 @@ void set_curr_task(int cpu, struct task_struct *p) /* task_group_lock serializes the addition/removal of task groups */ static DEFINE_SPINLOCK(task_group_lock); +/* + * Make sure that the task_group structure is cacheline aligned when + * fair group scheduling is enabled. + */ +#ifdef CONFIG_FAIR_GROUP_SCHED +static inline struct task_group *alloc_task_group(void) +{ + return kmem_cache_alloc(task_group_cache, GFP_KERNEL | __GFP_ZERO); +} + +static inline void free_task_group(struct task_group *tg) +{ + kmem_cache_free(task_group_cache, tg); +} +#else /* CONFIG_FAIR_GROUP_SCHED */ +static inline struct task_group *alloc_task_group(void) +{ + return kzalloc(sizeof(struct task_group), GFP_KERNEL); +} + +static inline void free_task_group(struct task_group *tg) +{ + kfree(tg); +} +#endif /* CONFIG_FAIR_GROUP_SCHED */ + static void free_sched_group(struct task_group *tg) { free_fair_sched_group(tg); free_rt_sched_group(tg); autogroup_free(tg); - kfree(tg); + free_task_group(tg); } /* allocate runqueue etc for a new task group */ @@ -7681,7 +7713,7 @@ struct task_group *sched_create_group(struct task_group *parent) { struct task_group *tg; - tg = kzalloc(sizeof(*tg), GFP_KERNEL); + tg = alloc_task_group(); if (!tg) return ERR_PTR(-ENOMEM); diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index efd3bfc..e679895 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -248,7 +248,12 @@ struct task_group { unsigned long shares; #ifdef CONFIG_SMP - atomic_long_t load_avg; + /* + * load_avg can be heavily contended at clock tick time, so put + * it in its own cacheline separated from the fields above which + * will also be accessed at each tick. + */ + atomic_long_t load_avg ____cacheline_aligned; #endif #endifI suppose the question is if it would be better to just move this to wind up on a separate cacheline without the extra empty space, though it would likely be more fragile and unclear.
I have been thinking about that too. The problem is anything that will be in the same cacheline as load_avg and have to be accessed at clock click time will cause the same contention problem. In the current layout, the fields after load_avg are the rt stuff as well some list head structure and pointers. The rt stuff should be kind of mutually exclusive of the CFS load_avg in term of usage. The list head structure and pointers don't seem to be that frequently accessed. So it is the right place to start a new cacheline boundary.
Cheers, Longman -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
